I had the same issue. The data often comes from the tax assessor's official records. Check (or have your Realtor check) the info on the assessor's site. Mine was incorrect, so I filed a "Property Data Change Request" form with them (Los Angeles County, in my case). They sent someone out to verify the information (4 baths instead of 3) and updated their records. It can take awhile for them to process it. When you list your home for sale with a Realtor, he/she will input the data, which may be different from the assessor's, into the MLS and this should be what is displayed on internet sites like Trulia. It is still best to get the info changed at the assessor's office, though.
Here is a link to the Los Angeles County form: http://ezforms.assessor.lacounty.gov/Form/Fill/28